Method of producing aluminum-magnesium sheet



United States Patent 3,347,714 METHQD 0F PRODUCING ALUMINUM- MAGNESIUM SHEET Irwin Broverman, Cheshire, and Michael I. Pryor, Hamden, Conn, assignors to Olin Mathieson Chemical Corporation, a corporation of Virginia N0 Drawing. Filed Dec. 27, 1963, Ser. No. 334,938 7 Claims. (Cl. 148-115) This invention relates to the achievement of improved physical properties in sheet and strip aluminum-magnesium alloys by the conjoint action of controlled cooling and pressure rolling. More particularly, it relates to a process for the development of desired physical properties of aluminum base alloys which are suitable for bright anodizing treatments by controlled cooling of the sheets or strips, and is directed toward the simplification of production of such material in a form suitable to the anodizing processing.

Of the many sheet metal products formed of rolled metal, a large percentage desirably have a surface appearance which is highly lustrous and generally pleasing to the eye, and also have functional metallurgical properties of hardness, formability, or other desirable combinations of physical properties. The production of sheet aluminum having these esthetic characteristics combined with physical properties is a highly developed art and constitutes a very substantial portion of the existing capacity of the aluminum industry.

In the art of producing sheet and strip material from aluminum containing small additions of magnesium which are suitable for bright anodizing, serious difliculties arise from the relationship of the combined factors of hot rolling temperature and pressure. For example, suitable and desirable surface, as well as bulk, characteristics in sheet and strip materials made from the aforementioned alloys is normally achieved through the conjoint acton of a moderate hot rolling temperature combined with an essentially high separating force on the hot mill rolls. The factor of temperature is critical because of the necessity of accomplishing the strain induced precipitation of intermetallic compounds such as Mg Si and fiMg Al at relatively low temperatures so that a very fine dispersed precipitate is obtained rather than at higher temperatures such as 700 to 800 F. where an undesirable coarse dispersion of these compounds is obtained.

In regard to the formation of these dispersions, it is well known that there are two types of constituents or intermetallic compounds which influence the bright anodizing characteristics of aluminum-magnesium alloys. Firstly, there are those which are nearly insoluble during anodizing and become included in the anodic film; these obviously reduce the light which is transmitted through the anodic film, and include such constituents as FeAl MnAl and aAlFeSi. Maximum bright anodizing characteristics are obtained if these constituents are present in a coarse particulate form.

The other type of constituents which influence bright anodizing characteristics are those which are soluble during anodizing such as Mg Si and Bphase Mg Al Their distribution affects the roughness of the reflecting anodic film-metal interface; but because they dissolve during anodizing they do not interfere with the oxide film clarity to a significant degree. A given quantity of these constituents dispersed in fine particulate form produces a smooth anodic film-metal interface having a high specular reflectivity. A similar quantity of these constituents dispersed in coarse particulate form produces a roughened oxide metal interface having a low specular reflectivity and poor image clarity.

It is also well known that the distribution of soluble constituents of Mg Si and Mg Al can be controlled in the hot rolling process. This is accomplished in an intermediate temperature range of 450 to F. under the conjoint action of temperature and roll deformation. The lower the temperature, the finer will be the dispersion of the constituent particles and therefore the higher the reflectivity after bright anodizing.

It is apparent that a reciprocal relationship exists between hot rolling temperature and roll separating force in that the lower the temperature at which hot rolling occurs, the less plasticity there is in the metal and consequently the greater is the separating force exerted on the rolling mills to achieve a given percentage reduction. One solution to this problem which has been proposed is to rapidly cool one side only of the sheet or strip in order to establish a thermal gradient between a surface layer of the sheet or strip and the remaining bulk of the material. When the strip is rolled while the thermal gradient exists, the desirable metallurgical characteristics necessary for subsequent bright anodizing are found to exist in the surface layer which has been cooled to below a predetermined temperature. On the other hand, roll separating force remains moderate since the bulk of the material at the moment of rolling is at a substantially higher temperature than the surface layer.

While this solution yields a product which is highly suitable for subsequent bright anodizing, there are many production problems which are not easily overcome. Firstly, such one side cooling is applicable only in light gages of the sheet or strip in view of the requirement that the cooling take place immediately prior to the final hot rolling reduction in order to prevent a reheating of the surface layer and consequent loss of the physical properties previously achieved. In addition, the coolant must be applied in extreme proximity to the roll bite in order to prevent complete loss of the thermal gradient as the result of heat conduction from the bulk of the material to the surface layer. Still further, production of sheet and strip material by this process is adversely affected due to the intimate relationship between the application of the coolant and the operation of the mill; thus, very precise controls are required to adapt the rate of coolant application to the rate of rolling mill operation.

Accordingly, it is desirable from the standpoint of economic feasibility to rapidly cool the entire thickness of the sheet or strip, or in some cases the ingot itself, down to a predetermined operating temperature and thereafter apply reducing roll pressure as needed at some temperature within this range in order to achieve the desirable characteristics and subsequent bright anodizing treatment through the bulk of the material. In addition to obviating the aforementioned difficulties encountered with one side cooling, bulk cooling achieves the desired physical characteristics through the thickness of the sheet. This condition may be beneficial in a variety of circumstances, but is particularly so where the surface of the finished sheet or strip is to be subjected to a mechanical surface finishing treatment such as bufling or burnishing. It has been found that the surface layer having characteristics necessary for subsequent bright anodizing as a result of one side cooling is so extremely thin that these mechanical surface treatments may completely remove the surface layer with heavy buffing or may remove the layer in certain areas due to uneven bulfing. In addition, rapid bulk cooling as distinguished from one side surface cooling permits a far greater degree of flexibility in line hot rolling operations, since, as will be seen more clearly hereinafter, bulk cooling is not limited to a particular location in the rolling mill operation in relation to the rolling mill line.

An additional unexpected result achieved by the practice of this invention is the attainment of higher mechanical properties throughout the thickness of the sheet or strip material. For example, it has been found that material fabricated in accordance with the process of this invention has yield strength and tensile strength values which are at least 2000 to 5000 psi. higher than corresponding values of material produced through processing which results in a coarse rather than a fine dispersion of the intermetallic compounds. It is to be understood that these values apply to material produced under the process of this invention, after which the material is cold rolled and partially annealed at about 400 to 500 F. to, for example H25 temper.

With these factors in mind, it is a principal object of this invention to provide an improved method of forming aluminum-magnesium alloy sheet or strip material which is suitable for bright anodizing.

It is another object of this invention to provide an improved method of forming aluminum-magnesium alloy sheet or strip material having metallurgical characteristics throughout the thickness of the material which are suitable for bright anodizing.

It is still another object of this invention to provide an improved method of forming aluminum-magnesium alloy sheet or strip material which greatly facilitates effective production processing to achieve a final product suitable for mechanical surface treatment and subsequent bright anodizing.

It is yet another object of the present invention to provide an improved method of producing aluminummagnesium alloy sheetor strip material in which certain metallurgical characteristics of the alloy are achieved through controlled quenching combined with plastic deformation within a specified temperature range at any stage in the hot rolling operation from ingot prior to final hot rolled gage strip.

These and other objects and advantages of the present invention will become apparent from a consideration of the following description.

In accordance with the principles .of this invention it has been found that an improved method of producing aluminum-magnesium alloy sheet or strip material, suitable for subsequent bright anodizing, in which the sheet has magnesium present in the form of at least one precipitated intermetallic compound uniformly dispersed in fine particle size, is achieved by providing a mass of aluminum-magnesium alloy having magnesium present in the form of at least one intermetallic magnesium compound as a soluble constituent thereof, dissolving the constituent into solid solution in the alloy, and precipitating the constituent throughout the thickness of the mass through the conjoint action of temperature and mechanical deformation to produce a fine particle size uniform dispersion of the constituent in the mass.

Aluminum alloys suitable for subsequent bright anodizing after being formed into sheet or strip material fall generally into two categories, those containing around .2% to 1.8% magnesium and those containing around 2% to 3.2% magnesium, both with Fe and Si as impurities in amounts less than .4%. Included within the first group are alloys 5257 (.2%.6%), 5357 (.8%-1.2%), 5457 (.8%1.2%), 5557 (.4%.8%), 5657 (.6% 1.80%), 5757 (.6%1.80%), 5857 (.5%.8%), 5957 (.4%.8%); among the second group are such alloys as 5252 (2.2%2.8%), 5652 (2.2%2.8%), and 5053 (3.2%). These aluminum-magnesium alloys contain magnesium in sufficient quantity to form intermetallic magnesium compounds which are soluble in high temperature ranges. In the case of the 2% to 1.8% magnesium group the operative intermetallic compound is,

Mg Si which is soluble in the temperature range of 750 to 900 F. and is precipitated upon cooling to below this temperature range. In the case of the 2% to 3.2% magnesium group two intermetallic compounds of magnesium are formed, Mg Si which is soluble in the temperature range previously stated, and Mg Al which is soluble in the temperature range of 850 to 950 F. and which precipitates upon subsequent cooling below this latter temperature range. In either case the lower temperature precipitation of these compounds is accelerated by deformation at lower temperatures.

After casting and scalping the ingot in conventional manner,,the alloy is reheated prior to breakdown hot rolling to a temperature of suflicient magnitude to red1s solve the intermetallic magnesium compound into solidsolution in the base metal. This temperature is within the range of 750 tor950 F. depending upon which of the aforementioned groups of aluminum-magnesium alloys is selected for processing. According to preferred practice, the process ingot is heated toapproximately 900 F. in order to drive the Mg Si into solid solution, or to approximately 950 R, which temperature is required to drive Mg AI into solid solution in the event that one to effect reduction of the ingot at lower temperatures.

Preferably, initial rapid cooling to the lower temperature range takes place subsequent to breakdown hot rolling by the prior to final reduction in the tandem mill. It is, of course, critical to the practice of this invention that the cooling take place at some intermediate stage prior to reduction to final gage in order to achieve the desired metallurgical characteristics obtained through the conjoint action of intermediate temperature hot rolling combined with plastic deformation.

In regard to the application of coolant. to effect a rapid transition from ingot or, breakdown hot rolling temperature to intermediate hot rolling temperature, it should be noted that the aluminum-magnesium alloys suitable for use in subsequent bright anodizing processing have an extremely high thermal conductivity in the order of B.t.u.s per hour per square foot per degree Fahrenas a high velocity jet which is directed against the sur face to be cooled substantially perpendicular thereto. Rapid cooling is achieved even though vaporization of the cooling liquid forms on thehot metal surface, thereby constituting a vaporous thermal barrier, due to the fact that the cooling medium moves at such a high velocity that it penetrates the barrier and brings the cooling liquid into direct contact with the high temperature surface.

Utilizing a quenching processes described above, it has been found that with an aluminous metal sheet or strip in the order of 3" to A", it is possible to achieve a temperature drop from about 950 F. to around 450 F. by using a spray system which removes heat at a coefficient of from 3,000 to 10,000 B.t.u.s per hour per square foot per degree Fahrenheit. To achieve this rate of heat removal, spray must be delivered at a volumetric The effect of this high pressure spray quenching to achieve an intermediate hot rolling temperature prior to deformation at this temperature is to retain the soluble constituents in supersaturated metastable solid solution, such that subsequently during rolling at the intermediate temperatures rapid precipitation of the intermetallic compound or compounds as the case may be is promoted as a fine particle size uniform dispersion throughout the thickness of the sheet or strip. In the case of the .2% to 1.8% magnesium alloys, temperatures of 600 F. or less, preferably within the range of 450 to 600 F., are required in order to promote the desired fine dispersion of Mg Si during subsequent rolling. Optimumly, the sheet or strip is reduced to a temperature within the range of 450 to 550 F. Subsequent rolling and deformation under these conditions will generate in the final sheet metallurgical characteristics ideal for good brightness after bright dipping and anodizing, with or without mechanical bufiing. Finally, in the case of the 2% to 3.2% magnesium group of alloys which contain not only Mg Si, which is precipitated in the above mentioned temperature ranges, but also Mg Al temperatures of 700 F. or less, preferably within the range of 550 to 700 F. are required in order to promote the desired fine dispersion of the latter compound during subsequent rolling. Again optimumly, this group of alloys is deformed within the range of 550 to 650 F. to achieve the desired results.

Precipitation of the intermetallic magnesium compounds as a uniform dispersion having fine particle size and small interparticle distance is effected through the conjoint action of moderate hot rolling temperature within the specified ranges and mechanical deformation of the ingot or sheet. Customarily, although not necessarily, this deformation is accomplished by pressure rolling of the ingot or sheet as soon after quenching as is practicable commensurate with the rolling mill and quenching facilities available. As a practical matter, the quenching operation may take place at any stage in the rolling operation from prior to initial breakdown rolling of the ingot to immediately preceding final hot reduction of an intermediate gage sheet or strip. Thus, the cast ingot may initially be quenched from the temperature necessary to dissolve the intermetallic magnesium compounds down to the lower temperatures described hereinabove, immediately after which the ingot is rolled continuously through intermediate gages to final hot reduction gage. It is readily apparent that one distinct disadvantage of this alternative lies in the extremely high forces which must be exerted upon the ingot to effect reduction in the lower temperature range.

A more preferred procedure is to subject the ingot to initial breakdown hot rolling from ingot size to an intermediate gage strip in the order of A1" to 3" in thickness, and then to quench the sheet at this gage from breakdown hot rolling temperature to the intermediate range temperatures coupled with subsequent rolling to final hot reduction gage at this intermediate temperature in order to precipitate the magnesium compounds in the desired fine particle size uniform dispersion. With this arrangement, less force is required of the breakdown rolling mills, in addition to which the quenching apparatus may be more effectively designed to operate over a larger surface area with less bulk thickness, consequently eliminating extreme design problems which would be encountered in an effort to achieve an equal degree of heat removal from an ingot exhibiting far less surface area and greater bulk thickness.

It is also possible to subject an ingot to initial breakdown hot rolling at the normal elevated temperature, quench to the intermediate temperature range, and thereafter pass the intermediate gage material again through the breakdown hot rolling mill to effect further reduction to another intermediate gage, and then pass this sheet through the final hot reduction mill.

For purposes of definition, distinction between fine and coarse particle size is made by defining fine particle size as that which is not resolvable under a conventional optical microscope at a magnification of 500 diameters.

The following are examples of the pr-atice of this invention which are to be deemed as illustrative thereof and not all inclusive.

Example I A 5457 aluminum-magnesium alloy was cast by the DC process into a 18" thick ingot. The cast ingot was then homogenized by heating at about 1025 F. for about 24 hours, cooled to room temperature and scal ed to remove surface imperfections. It was then reheated to 900 F., and reduced in a reversing mill from approximately 17" to approximately A. slab. The slab was then passed through a high pressure spray quenching apparatus at a linear speed about ft. per minute. The slab was subjected to a plurality of high velocity streams of a cooling medium at a pressure of 250- p.s.i. to provide a slab temperature of 550 F. upon emerging from the quenching apparatus. The cooling medium nozzles were located about one foot away from the surface of the strip and directed at right angles thereto. The slab at the 550 F. temperature passed into the final hot rolling mill and was subjected to further reductions to about .160" to precipitate the intermetallic compound under the conjoint action of intermediate temperature and mechanical deformation. Upon examination the as hot rolled sheet was found to have a fine particle size uniform dispersion of Mg Si intermetallic compound distributed throughout the thickness of the sheet.

Example II A sample of 5252 aluminum alloy was prepared and processed in the manner of Example I with the exceptions that a temperature of about 1000 F. was used to homogenize the as cast ingot and the initial breakdown hot rolling was carried out at 950 F. The bulk temperature of the strip immediately following high pressure spray quenching was 600 F. The desired results were obtained.

It will be apparent from the foregoing description that there has been provided a method of producing a. bright anodizing aluminum-magnesium sheet which is believed to provide a solution to the foregoing problems and achieve the aforementioned objects. It is to be understood that the invention is not limited to the examples described herein which are deemed to be merely illustrative of the best modes of carrying out the invention, but rather is intended to encompass all such modifications as are within the spirit and scope of the invention as set forth in the appended claims.

What is claimed and desired to be Patent is:

1. A method of producing aluminum-magnesium sheet having magnesium present in the form of at least one precipitated intermetallic compound uniformly dispersed in fine particle size throughout the bulk of said sheet, said method comprising the steps of:

(A) providing a mass of aluminum-magnesium alloy having magnesium present in the form of at least one intermetallic compound as a soluble constituent of said alloy, said alloy containing from 0.2 to 3.2% magnesium, balance essentially aluminum,

(B) heating said mass to a temperature within the range of 750 to 950 F. thereby dissolving said constituent into solid solution in said alloy,

(C) rapidly cooling said mass at a rate which removes heat at a coeflicient of from 3,000 to 10,000 B.t.u.s per hour per square foot per degree Fahrenheit to a temperature within the range of 450 to 700 F., and

(D) pressure rolling said mass to effect a reduction in the thickness thereof while said mass is at said cooled temperature, whereby said constituent is precipitated throughout the reduced thickness of said mass as a secured by Letters 7 fine particle size uniform dispersion of said constituent.

2. The method as set forth in claim 1 wherein (A) said magnesium is present in the form of Mg Si as said soluble constituent,

(B) said heating is within the range of 750 to 900 F., and (C) said rapid cooling to 550 F.

3. The method as set forth in claim 1 wherein (A) said magnesium is present in the. form of Mg Si and Mg Al as soluble constituents of said alloy,

(B) said heating is within the range of 850 to 950 F.,

and

(C) said rapid cooling is within the range of 550 to 4. A method of producing aluminum-magnesium sheet having magnesium present in the form of at least one precipitated intermetallic compound uniformly dispersed in fine particle size throughout the bulk of said sheet, said method comprising the steps of (A) providing an ingot of aluminum-magnesium alloy having magnesium present in the form of at least one intermetallic compound as a soluble constituent is within the range of 450 of said alloy, said alloy containing from 0.2 to 3.2%

magnesium, balance essentially aluminum,

(B) heating said ingot substantially uniformly throughout the thickness thereof to a temperature within the range of 750 to 950 F. thereby dissolving said constituent into solid solution in said alloy,

(C) rapidly cooling said ingot throughout the thickness thereof at a rate which removes heat at a coefficient of from.3,000 to 10,000 B.t.u.s per hour per square foot per degree Fahrenheit to a temperature within the range of 450 to 700 F., and

(D) pressure rolling said ingot to sheet while said ingot is at said lower temperature whereby said constituent is precipitated throughout the thickness of said sheet as a fine particle size uniform dispersion of said constituent.

5. The method as set forth in claim 4 wherein said rapid cooling is achieved by quenching said ingot under a plurality of high pressure liquid sprays directed around the surface of said ingot.

6. A method of producing aluminum-magnesium sheet having magnesium present in the form of at least one precipitated intermetallic compound uniformly dispersed in fine particle size throughout the bulk of said sheet, said method comprising the steps of:

(A) providing an ingot of aluminum-magnesium alloy having magnesium present in the form of at least one intermetallic compound as a soluble constituent of said alloy, said alloy containing from 0.2 to 3.2% magnesium, balance essentially aluminum,

(B) heating said ingot substantially uniformly throughout the thickness thereof to a temperature within the range of 750 to 950 F. thereby dissolving said constituent into solid solution in said alloy,

(C) pressure rolling said ingot to an intermediate gage sheet while. said ingot is within said temperature range,

(D) rapidly cooling said intermediate gage sheet throughout the thickness thereof at a rate which removes heat at a coefficient of from 3,000 to 10,000 B.t.u.s per hour per square foot per degree Fahrenheit to. a temperature within the range of 450 to' 700 F., and

(E) pressure rolling said intermediate gage sheet to final gage sheet while said sheet is at said lower temperature whereby said constituent is precipitated throughout the thickness of said sheet as a fine particle size uniform dispersion of said constituent.

7. The method as set forth in claim 6 wherein said rapid cooling is achieved by quenching said sheet under a plurality of high pressure liquid sprays directed against the surfaces of said sheet.

References Cited UNITED STATES PATENTS 6/1965v English 14811.5, X 2/1966 Pryor 148 UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3,347,714 October 17, 1967 Irwin Broverman et a1.

It is certified that error appears in the above identified patent and that said Letters Patent are hereby corrected as shown below:

Column 1, line 38, "acton" should read action Column 2, line 4, "70 F." should read 700 F.

Signed and sealed this 23rd day of December 1969.

(SEAL) Attest:

Edward M. Fletcher, Jr.

Attesting Officer Commissioner of Patents WILLIAM E. SCHUYLER, JR. 

1. A METHOD OF PRODUCING ALUMINUM-MAGNESIUM SHEET HAVING MAGNESIUM PRESENT IN THE FORM OF AT LEAST ONE PRECIPITATED INTERMETALLIC COMPOUND UNIFORMLY DISPERSED IN FINE PARTICLE SIZE THROUGHOUT THE BULK OF SAID SHEET, SAID METHOD COMPRISING THE STEPS OF: (A) PROVIDING A MASS OF ALUMINUM-MAGNESIUM ALLOY HAVING MAGNESIUM PRESENT IN THE FORM OF AT LEAST ONE INTERMETALLIC COMPOUND AS A SOLUBLE CONSTITUENT OF SAID ALLOY, SAID ALLOY CONTAINING FROM 0.2 TO 3.2% MAGNESIUM, BALANCE ESSENTIALLY ALUMINUM, (B) HEATING SAID MASS TO A TEMPERATURE WITHIN THE RANGE OF 750* TO 950*F. THEREBY DISSOLVING SAID CONSTITUENT INTO SOLID SOLUTION IN SAID ALLOY, (C) RAPIDLY COOLING SAID MASS AT A RATE WHICH REMOVES HEAT AT A COEFFICIENT OF FROM 3,000 TO 10,000 B.T.U.''S PER HOUR PER SQUARE FOOT PER DEGREE FAHRENHEIT TO A TEMPERATURE WITHIN THE RANGE OF 450* TO 700*F., AND (D) PRESSURE ROLLING SAID MASS TO EFFECT A REDUCTION IN THE THICKNESS THEREOF WHILE SAID MASS IS AT SAID COOLED TEMPERATURE, WHEREBY SAID CONSTITUENT IS PRECIPITATED THROUGHOUT THE REDUCED THICKNESS OF SAID MASS AS A FINE PARTICLE SIZE UNIFORM DISPERSION OF SAID CONSTITUENT. 